Silica sand is the most widely used aggregate in the foundry industry. Its low cost, due to its abundance, makes it an attractive option to metal casters. However, steel and iron castings in silica sand molds tend to exhibit defects such as veining, fins and dimensional inaccuracy. This is, in part, due to the large thermal expansion of silica sand. Previous studies into the high temperature properties of silica sand have addressed the technical limitations metal casters face while using silica sand molds or cores. Silica sand undergoes various phase transitions while being heated up to high temperatures. Once past the alpha-beta phase transition at approximately 570° C. (1058 F), silica sand experiences a steady contraction till the cristobalite phase transition at 1470° C. (2678 F). Various sand additives such as iron oxide or Engineered Sand Additives (ESA) are used in the metal casting industry to either induce a tridymite transition, which leads to a secondary expansion, or induce the cristobalite transition at a lower temperature, which causes a large secondary expansion. These additives cause large changes in the volume of bonded sand.
Veining defects in silica sand are caused by the loss of strength on the surface of the cores, which leads to a network of cracks arising from the high thermal expansion of the sand. These cracks are then filled by the liquid metal, which breakouts thereby form veins on the surface of the casting. Certain additives promote the sintering of the surface of the core and form a partially melted surface. This causes an increase in the rigidity of the surface due to the increase in the viscosity of the sintered surface. The increase in viscosity at higher temperatures leads to higher strengths on the surface of the core, resulting in reduced core distortion. The alpha-beta transition in silica sand is associated with a high peak expansion which causes dimensional inaccuracy in steel castings. The dimensional accuracy of castings depends on various factors such as section thickness of the casting, temperature and expansion and does not exhibit a linear trend as per the patternmaker's shrink rule.
Due to the above mentioned limitations of silica sand, certain specialty aggregates such as zircon or chromite are used as core materials for steel castings. Both these aggregates have low thermal expansion when compared to silica sand. They also have higher refractory values. These properties result in a lower core distortion, thereby leading to more dimensionally accurate castings. A major limitation, however, on the use of specialty aggregates is their relatively higher cost, and their use is thus usually confined to certain casting applications that unavoidably require the use of 100% specialty sands. For most other applications, a small improvement in the high temperature properties of silica sand will result in good casting quality and dimensional accuracy.
It is an object of the present invention to address the difficulties encountered with silica sands as a foundry sand in a manner that achieves improved castings at an acceptable cost.
As used herein, except where the context requires otherwise, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised”, are not intended to exclude further additives, components, integers or steps.